It has been shown that the line width roughness (LWR) and line edge roughness (LER) of the features of a device have an impact on the device performance. As the critical dimensions (CD) of devices shrink to 35 nm and below, control and monitoring of LWR and LER become increasingly more critical. For example, nano-scale LER and LWR have a significant impact on transistor performance and on the performance and reliability of advanced interconnects. Specifically, the roughness on the sidewall of a dielectric feature of a back end of line (BEOL) structure will transfer to a copper interconnect at the interface. As a result, the copper conductivity may be adversely affected because of the enhanced electron scattering. There are currently several viable techniques that measure LWR and LER, but most of them cannot be used in-situ with existing process control metrology tool.
Thus, there is need in the art, for a fast, nondestructive, noninvasive, and production worthy metrology tool, which is capable of meeting the process monitoring requirements for 32 nm nodes and beyond.
In prior art, attempts were made to measure LER and LWR by optical methods in dark-field, or combination of dark and bright field modes. (Microelectronic Engineering 84, 619 (2007); Proc. SPIE vol. 5752, 192 (2005)). However, the prior art fails to teach that the diffuse scattering depends on the roughness and the feature shapes and dimensions, and not just on the roughness alone. Thus the prior art does not provide sufficient teaching for quantitative interpretation of the signal in terms of the roughness.